In cable modem and cellular systems and other wireless technologies where digital data is transmitted from one place to another using modulated radio frequency carriers there is always a problem of impulse noise corrupting the transmissions and causing errors. Impulse noise can be caused by many sources.
Digital data transmissions typically involve, a stream of data bits which are broken down into code words such as Reed-Solomon blocks for encoding with error detection and correction bits (hereafter EOC bits).
Impulse noise detection in the time domain is known and is in commercial use in the Cable Modem Termination System (hereafter CMTS) receivers code named Jasper I supplied by the assignee of the present invention. The time domain impulse detection technique used a window that was several samples wide and detected the power received during that window and compared it to a threshold. The CMTS circuitry controls the transmitted power of each burst by downstream messages, so it knows what power to expect. The threshold was set high enough so that data transmissions not corrupted by impulse noise would not have sufficient power to exceed the threshold. If the threshold was exceeded during the window, it was assumed that impulse noise had an additive effect on the payload data to corrupt it and exceed the noise threshold. This time domain system can discover high power impulse noise and take steps to minimize its effect by setting an erasure bit on any symbol in the window that may be corrupted by noise.
However, with lower power impulse noise, the detection problem becomes much more difficult because it is much more difficult to distinguish between what is noise and what is real signal, and to eliminate only the noise. This is because the use of a power threshold is limited, and when it is lowered enough to detect low power noise, the power of uncorrupted payload data may also exceed the threshold and cause false erasure indications of symbols that are not corrupted by noise. There is a limit to how close to the expected power the noise power threshold can be set. This allows some symbols corrupted by low power noise to get through the system and can lead to errors in recovering the payload data and throw the CMTS tracking loops off synchronization.
Therefore, a need has arisen for a way of detecting low power or high power impulse noise without giving erasure indications on symbols that are not corrupted by noise.